This invention relates generally to writing materials, and more particularly to a novel writing material which develops color when used on a surface having electron acceptors.
Heretofore, various solid vehicles such as waxes, polymers, and the like have been used for supporting pigment and forming writing materials such as crayons and "crepas". However, if such a solid vehicle supports, in a dispersed state, a non-volatile oily solvent containing an electron donative chromogenic compound of a nature developing color upon contact with an electron accepting material, it will acquire various disadvantageous features, such as the liquid substance being diffused onto the outer surface of the writing materials, thereby causing the writing material to become wet or to perspire, or the surface of the writing material to be tinted, or bringing about a deterioration of the color developing nature of the writing material at the time of writing.
When the non-voltile oily solvent is thus substantially lost or the amount thereof is much reduced, the electron transferring reaction caused at the time when the writing material comprising the electron donative chromogenic compound, non-volatile oily solvent, and the solid vehicle for supporting the compound and the solvent, is brought into contact with a surface having electron acceptors, is substantially lowered regardless of the existence of sufficient electron donative compound and electron acceptors at the contacting portion of the surface, and a considerably long time is required for developing color, or a desired density of color cannot be obtained at all.
For developing color of a high density in a short period, it is required that a solvent supporting the chromogenic compound exist in the writing material in a range of from 10 to 50 % by weight of the writing material. The non-volatile oily solvent is used for this purpose. However, it is also essential that the electron donative chromogenic compound be dissolved or dispersed in the solvent without developing color on the surface of the writing material, and that the oily solvent be substantially non-volatile at the room temperature.
As for the solvent satisfying these requirements, the solvents of high boiling points such as esters of organic acids, aromatic hydrocarbons, chlorinated polyphenyls, phosphoric acid esters, and the like can be used. However, most preferable of the solvents are non-poisonous esters of organic acids such as methyl acetyl ricinolate, dibutyl phthalate, phthalyl butyl glycolate, and aromatic hydrocarbon solvents, which have a molecular weight of from 150 to 300, and include at least one aromatic ring in the molecule, and the boiling point of which is higher than 500.degree.F. Furthermore, any of the solvents which has somewhat higher vapor pressure than the non-volatile oily solvents and dissolves the electron donative chromogenic substance sufficiently, and which comprises, for instance, an ester of salicylic acid, of benzoic acid, and benzyl alcohol, may be used as an auxiliary solvent to assist the non-volatile oily solvent.
In the case where from 10 to 50 % by weight of non-volatile oily solvent and ordinary solid vehicles or fillers such as waxes, polymers, clay, and precipitated inorganic salts are used in the writing material, there is a possibility of the liquid compound being diffused out of the writing material and the surface of the latter becoming wet or perspiring, thereby becoming sticky.